Magnetic resonance imaging (MRI) is a medical patient diagnostic technology for producing high-quality images noninvasively. When MRI is combined with magnetic resonance spectroscopy (MRS), a chemical shift spectrum within each MRI pixel or, equivalently, images of different chemical species can be obtained, allowing more detailed studies of the subject. This procedure is termed spectroscopic imaging or chemical shift imaging (CSI) and has a great diagnostic potential.
One major practical problem of CSI is its very long data acquisition time, as compared to ordinary MRI. The long acquisition time is due to the additional chemical shift dimension to be resolved using a Fourier transform. For example, CSI can take 200 to 300 times longer than a standard MRI to achieve the same spatial resolution. Thus, a three-minute scan can become 13 hours long. In order to achieve practical scan times, CSI usually is performed with very low spatial resolution. Unfortunately, low spatial resolution may degrade the spectral resolution due to larger magnetic field (B.sub.0) inhomogeneity per pixel, partial volume, and truncation artifacts. This results in difficulty in identifying and quantifying each chemical shift peak in the spectrum of each pixel. In addition, currently available CSI data processing is complex, time consuming, and somewhat unreliable.
To address the limitations of CSI in the context of water-fat imaging, Dixon introduced a method of modeling the proton spectrum of water-fat in tissue as two delta functions in W. T. Dixon, "Simple Proton Spectroscopic Imaging", Radiology, 153, 189-194 (1984). Dixon employed shifted 180.degree. RF pulses, or echo-times (TEs), to put the magnetization vectors of water and fat parallel and antiparallel, respectively, in a pair of images. Simple addition and subtraction of the two images in complex form yield separate images of the two chemical components, i.e., water and fat, provided that phase errors are negligible. Unfortunately, phase errors are always present in a realistic magnetic resonance image.
In order to overcome the phase error limitations of the Dixon method, the inventors have previously proposed a method of water-fat imaging with direct phase encoding (DPE). This method uses three general acquisition points to acquire three complex images, and provides two sets of general solutions of the two chemical components, namely, water and fat. The method resolves the image by selecting the correct set of solutions for each pixel, thus identifying water and fat. This application of the DPE method has been limited to water-fat imaging. A need exists for a simple but novel method to greatly improve, in both data acquisition and data processing, the efficiency of CSI having general applicability.